Endothelial nitric oxide synthase is a site of superoxide synthesis in endothelial cells treated with glyceryl trinitrate

Sodium nitrite induces tolerance in the mouse aorta: Involvement of the renin-angiotensin system, nitric oxide synthase, and reactive oxygen species

Nitrites have emerged as promising therapeutic agents for cardiovascular diseases, alongside nitrates. While chronic use of organic nitrates is well recognized to lead to vascular tolerance, the tolerance associated with nitrite therapy remains incompletely understood. The aim of the present study was to investigate vascular tolerance to sodium nitrite and the underlying molecular mechanisms. Endothelium-denuded aortic rings isolated from male Balb/C mice were incubated with either the EC50 (10-4 mol/L) or EC100 (10-2 mol/L) concentration of sodium nitrite for 15 min to induce tolerance. The EC100 concentration of sodium nitrite induced vascular tolerance. Pre-incubation with captopril and losartan effectively reversed sodium nitrite-induced tolerance. Similarly, pre-incubation with L-NAME and L-arginine prevented sodium nitrite-induced tolerance. Increased levels of reactive oxidative species (ROS) and reduced bioavailability of nitric oxide (NO) were observed in tolerant aortas. Increased superoxide dismutase (SOD) activity and decreased catalase activity were also verified in tolerant aortas. Both captopril and L-NAME prevented the increased levels of ROS observed in tolerant aortas. Furthermore, pre-incubation with catalase effectively prevented sodium nitrite-induced tolerance. Our findings suggest that sodium nitrite induces vascular tolerance through a signaling pathway involving the renin-angiotensin system, nitric oxide synthase, and ROS. This study contributes to the understanding of the interaction between nitrites and vascular tolerance and highlights potential targets to overcome or prevent this phenomenon.

Endothelial dysfunction due to the inhibition of the synthesis of nitric oxide: Proposal and characterization of an in vitro cellular model

The vascular endothelium plays a pivotal role in the maintenance of vascular homeostasis, mediated by vasoactive molecules produced by endothelial cells. The balance between vasoconstrictor and vasodilator biomolecules is what guarantees this equilibrium. Therefore, an increase in the bioavailability of vasoconstrictors along with a reduction in vasodilators may indicate a condition known as endothelial dysfunction. Endothelial dysfunction is marked by an inflammatory process and reduced activity of vasoprotective enzymes, being characterized by some factors like the reduction of the bioavailability of nitric oxide (NO) and increase in the production of reactive oxygen species (ROS), pro-inflammatory and vasoconstrictor molecules. This condition is a predictive marker of several cardiovascular diseases (e.g., atherosclerosis, hypertension, and diabetes). Research is affected by the scarcity of suitable in vitro models that simulate endothelial dysfunction. The goal of this study was to induce an in vitro condition to mimic endothelial dysfunction by inhibiting NO synthesis in cells. Thymus-derived endothelial cells (tEnd.1) were treated with different concentrations of L-NAME (from 1 to 1,000 μM) for different times (12, 24, 48, 72, 96, and 120 h without and with retreatment every 24 h). Cell viability, nitrite concentration, p22phox, NOX2, NOX4, IL-6, and ACE genes expression and lipid peroxidation were evaluated. The results indicate that the treatment with 100 μM L-NAME for 72 h without retreatment reduced NO concentration and NOX4 gene expression while increasing ACE expression, thus mimicking reduced vascular protection and possibly increased vasoconstriction. On the other hand, treatment with 100 μM L-NAME for 96 h with retreatment reduced the concentration of NO and the expression of the p22phox gene while increasing the expression of the IL-6 and ACE genes, mimicking the increase in inflammation and vasoconstriction parameters. Based on these results, we thus propose that both 100 μM L-NAME for 72 h without retreatment and 100 μM L-NAME for 96 h with retreatment may be used as models for in vitro endothelial dysfunction according to the purpose of the study to be conducted.

Nitric Oxide Is the Cause of Nitroglycerin Tolerance: Providing an Old Dog New Tricks for Acute Heart Failure

Our paper highlights the past 50 years of research focusing solely on tolerance involving nitroglycerin (glyceryl trinitrate, GTN). It also identifies and discusses inconsistencies in previous mechanistic explanations that have failed to provide a way to administer GTN continuously, free of limitations from tolerance and without the requirement of a nitrate-free interval. We illustrate, for the first time in 135 years, a mechanism whereby nitric oxide, the mediator of vasodilation by GTN, may also be the cause of tolerance. Based on targeting superoxide from mitochondrial complex I, uncoupled by glutathione depletion in response to nitric oxide from GTN, a novel unit dose GTN formulation in glutathione for use as a continuous i.v. infusion has been proposed. We hypothesize that this will reduce or eliminate tolerance seen currently with i.v. GTN. Finally, to evaluate the new formulation we suggest future studies of this new formulation for the treatment of acute decompensated heart failure.

Treating Acute Decompensated Heart Failure in Patients with COVID-19 Using Intravenous Nitroglycerin in 5% Glutathione

The purpose of this current opinion article is to illustrate a novel approach to the treatment of acute decompensated heart failure (ADHF) in coronavirus disease 2019 (COVID-19) patients. The approach described herein relies on a reformulation of intravenous nitroglycerin in 5% glutathione, itself novel, and is felt to have the potential to not only improve the rate of resolution of ADHF, but also reduce the risk of complications of heart failure seen in patients with COVID-19.

Nitric oxide (NO) synthase but not NO, HNO or H2 O2 mediates endothelium-dependent relaxation of resistance arteries from patients with cardiovascular disease

BACKGROUND AND PURPOSE

Superoxide anions can reduce the bioavailability and actions of endothelium-derived NO. In human resistance-sized arteries, endothelium-dependent vasodilatation can be mediated by H₂ O₂ instead of NO. Here, we tested the hypothesis that in resistance arteries from patients with cardiovascular disease, endothelium-dependent vasodilatation is mediated by a reactive oxygen species and not impaired by oxidative stress.

EXPERIMENTAL APPROACH

Small arteries were isolated from biopsies of the parietal pericardium of patients undergoing elective cardiothoracic surgery and were studied using immunohistochemical and organ chamber techniques.

KEY RESULTS

NO synthases 1, 2 and 3, superoxide dismutase 1 and catalase proteins were observed in the microvascular wall. Relaxing responses to bradykinin were endothelium dependent. During submaximal depolarization-induced contraction, bradykinin-mediated relaxations were inhibited by inhibitors of NO synthases (NOS) and soluble guanylyl cyclase (sGC) but not by scavengers of NO or HNO, inhibitors of cyclooxygenases, neuronal NO synthase, superoxide dismutase or catalase, or by exogenous catalase. During contraction stimulated by endothelin-1, these relaxations were not reduced by any of these interventions except DETCA, which caused a small reduction.

CONCLUSION AND IMPLICATIONS

In resistance arteries from patients with cardiovascular disease, endothelium-dependent relaxations seem not to be mediated by NO, HNO or H₂ O₂ , although NOS and sGC can be involved. These vasodilator responses continue during excessive oxidative stress.

Endothelial dysfunction and platelet hyperactivity in type 2 diabetes mellitus: molecular insights and therapeutic strategies

The incidence and prevalence of diabetes mellitus is rapidly increasing worldwide at an alarming rate. Type 2 diabetes mellitus (T2DM) is the most prevalent form of diabetes, accounting for approximately 90-95% of the total diabetes cases worldwide. Besides affecting the ability of body to use glucose, it is associated with micro-vascular and macro-vascular complications. Augmented atherosclerosis is documented to be the key factor leading to vascular complications in T2DM patients. The metabolic milieu of T2DM, including insulin resistance, hyperglycemia and release of excess free fatty acids, along with other metabolic abnormalities affects vascular wall by a series of events including endothelial dysfunction, platelet hyperactivity, oxidative stress and low-grade inflammation. Activation of these events further enhances vasoconstriction and promotes thrombus formation, ultimately resulting in the development of atherosclerosis. All these evidences are supported by the clinical trials reporting the importance of endothelial dysfunction and platelet hyperactivity in the pathogenesis of atherosclerotic vascular complications. In this review, an attempt has been made to comprehensively compile updated information available in context of endothelial and platelet dysfunction in T2DM.

Fasudil ameliorates endothelial dysfunction in streptozotocin-induced diabetic rats: a possible role of Rho kinase

Endothelial dysfunction is a major contributor to the pathogenesis of vascular disease in diabetes mellitus and RhoA/Rho-kinase (ROCK) system appears to play a crucial role in this setting. The present study was conducted to investigate the effect of the selective ROCK inhibitor, fasudil, on diabetes-related endothelial dysfunction and elucidated its underlying mechanism(s). Diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ, 50 mg/kg), and fasudil (5 mg/kg per day) was orally administered for 8 weeks. Our results showed that fasudil administration attenuated the increased activity/expression of ROCK (627.5 ± 27 vs. 247.8 ± 19.1) and the NADPH oxidase subunits, NOX2 and p47phox, in diabetic rat aorta. Fasudil could reduce the elevated tumor necrosis factor (TNF)-α (70.2 ± 14.1 vs. 25.3 ± 5.2) and transforming growth factor (TGF-β) levels and restored the deficit in antioxidant level of the diabetic aorta. Additionally, fasudil markedly improved the endothelial dysfunction in the diabetic aorta (73.8 ± 8.1 vs. 47.42 ± 8.69) and corrected the dysregulated endothelial nitric oxide (eNOS) expression. In conclusion, the present study demonstrates that fasudil effectively ameliorates the endothelial dysfunction in STZ-induced diabetic rats through inhibition of the Rho/ROCK pathway and thereby reducing the TNF-α-mediated NADPH oxidase activation.

Deregulation of arginase induces bone complications in high-fat/high-sucrose diet diabetic mouse model

A balanced diet is crucial for healthy development and prevention of musculoskeletal related diseases. Diets high in fat content are known to cause obesity, diabetes and a number of other disease states. Our group and others have previously reported that activity of the urea cycle enzyme arginase is involved in diabetes-induced dysregulation of vascular function due to decreases in nitric oxide formation. We hypothesized that diabetes may also elevate arginase activity in bone and bone marrow, which could lead to bone-related complications. To test this we determined the effects of diabetes on expression and activity of arginase, in bone and bone marrow stromal cells (BMSCs). We demonstrated that arginase 1 is abundantly present in the bone and BMSCs. We also demonstrated that arginase activity and expression in bone and bone marrow is up-regulated in models of diabetes induced by HFHS diet and streptozotocin (STZ). HFHS diet down-regulated expression of healthy bone metabolism markers (BMP2, COL-1, ALP, and RUNX2) and reduced bone mineral density, bone volume and trabecular thickness. However, treatment with an arginase inhibitor (ABH) prevented these bone-related complications of diabetes. In-vitro study of BMSCs showed that high glucose treatment increased arginase activity and decreased nitric oxide production. These effects were reversed by treatment with an arginase inhibitor (ABH). Our study provides evidence that deregulation of l-arginine metabolism plays a vital role in HFHS diet-induced diabetic complications and that these complications can be prevented by treatment with arginase inhibitors. The modulation of l-arginine metabolism in disease could offer a novel therapeutic approach for osteoporosis and other musculoskeletal related diseases.

A mitochondrial superoxide theory for oxidative stress diseases and aging

Fridovich identified CuZnSOD in 1969 and manganese superoxide dismutase (MnSOD) in 1973, and proposed ”the Superoxide Theory,” which postulates that superoxide (O₂^•−) is the origin of most reactive oxygen species (ROS) and that it undergoes a chain reaction in a cell, playing a central role in the ROS producing system. Increased oxidative stress on an organism causes damage to cells, the smallest constituent unit of an organism, which can lead to the onset of a variety of chronic diseases, such as Alzheimer’s, Parkinson’s, amyotrophic lateral sclerosis and other neurological diseases caused by abnormalities in biological defenses or increased intracellular reactive oxygen levels. Oxidative stress also plays a role in aging. Antioxidant systems, including non-enzyme low-molecular-weight antioxidants (such as, vitamins A, C and E, polyphenols, glutathione, and coenzyme Q₁₀) and antioxidant enzymes, fight against oxidants in cells. Superoxide is considered to be a major factor in oxidant toxicity, and mitochondrial MnSOD enzymes constitute an essential defense against superoxide. Mitochondria are the major source of superoxide. The reaction of superoxide generated from mitochondria with nitric oxide is faster than SOD catalyzed reaction, and produces peroxynitrite. Thus, based on research conducted after Fridovich’s seminal studies, we now propose a modified superoxide theory; i.e., superoxide is the origin of reactive oxygen and nitrogen species (RONS) and, as such, causes various redox related diseases and aging.

Nitroglycerine-induced nitrate tolerance compromises propofol protection of the endothelial cells against TNF-α: the role of PKC-β2 and NADPH oxidase

Continuous treatment with organic nitrates causes nitrate tolerance and endothelial dysfunction, which is involved with protein kinase C (PKC) signal pathway and NADPH oxidase activation. We determined whether chronic administration with nitroglycerine compromises the protective effects of propofol against tumor necrosis factor (TNF-) induced toxicity in endothelial cells by PKC-β₂ dependent NADPH oxidase activation. Primary cultured human umbilical vein endothelial cells were either treated or untreated with TNF-α (40 ng/mL) alone or in the presence of the specific PKC-β₂ inhibitor CGP53353 (1 μM)), nitroglycerine (10 μM), propofol (100 μM), propofol plus nitroglycerin, or CGP53353 plus nitroglycerine, respectively, for 24 hours. TNF-α increased the levels of superoxide, Nox (nitrate and nitrite), malondialdehyde, and nitrotyrosine production, accompanied by increased protein expression of p-PKC-β₂, gP91phox, and endothelial cell apoptosis, whereas all these changes were further enhanced by nitroglycerine. CGP53353 and propofol, respectively, reduced TNF-α induced oxidative stress and cell toxicity. CGP53353 completely prevented TNF-α induced oxidative stress and cell toxicity in the presence or absence of nitroglycerine, while the protective effects of propofol were neutralized by nitroglycerine. It is concluded that nitroglycerine comprises the protective effects of propofol against TNF-α stimulation in endothelial cells, primarily through PKC-β₂ dependent NADPH oxidase activation.

Arginase II inhibition prevents nitrate tolerance

BACKGROUND AND PURPOSE

Nitrate tolerance, the loss of vascular responsiveness with continued use of nitrates, remains incompletely understood and is a limitation of these therapeutic agents. Vascular superoxide, generated by uncoupled endothelial NOS (eNOS), may play a role. As arginase competes with eNOS for L-arginine and may exacerbate the production of reactive oxygen species (ROS), we hypothesized that arginase inhibition might reduce nitrate tolerance.

EXPERIMENTAL APPROACH

Vasodilator responses were measured in aorta from C57Bl/6 and arginase II knockout (argII -/-) mice using myography. Uncoupling of eNOS, determined as eNOS monomer : dimer ratio, was assessed using low-temperature SDS-PAGE and ROS levels were measured using L-012 and lucigenin-enhanced chemiluminescence.

KEY RESULTS

Repeated application of glyceryl trinitrate (GTN) on aorta isolated from C57Bl/6 mice produced a 32-fold rightward shift of the concentration-response curve. However this rightward shift (or resultant tolerance) was not observed in the presence of the arginase inhibitor (s)-(2-boronethyl)-L-cysteine HCl (BEC; 100 µM) nor in aorta isolated from argII -/- mice. Similar findings were obtained after inducing nitrate tolerance in vivo. Repeated administration of GTN in human umbilical vein endothelial cells induced uncoupling of eNOS from its dimeric state and increased ROS levels, which were reduced with arginase inhibition and exogenous L-arginine. Aortae from GTN tolerant C57Bl/6 mice exhibited increased arginase activity and ROS production, whereas vessels from argII -/- mice did not.

CONCLUSION AND IMPLICATIONS

Arginase II removal prevents nitrate tolerance. This may be due to decreased uncoupling of eNOS and consequent ROS production.

Endothelial dysfunction and diabetes: effects on angiogenesis, vascular remodeling, and wound healing

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by inappropriate hyperglycemia due to lack of or resistance to insulin. Patients with DM are frequently afflicted with ischemic vascular disease or wound healing defect. It is well known that type 2 DM causes amplification of the atherosclerotic process, endothelial cell dysfunction, glycosylation of extracellular matrix proteins, and vascular denervation. These complications ultimately lead to impairment of neovascularization and diabetic wound healing. Therapeutic angiogenesis remains an attractive treatment modality for chronic ischemic disorders including PAD and/or diabetic wound healing. Many experimental studies have identified better approaches for diabetic cardiovascular complications, however, successful clinical translation has been limited possibly due to the narrow therapeutic targets of these agents or the lack of rigorous evaluation of pathology and therapeutic mechanisms in experimental models of disease. This paper discusses the current body of evidence identifying endothelial dysfunction and impaired angiogenesis during diabetes.

The role of RhoA/Rho kinase pathway in endothelial dysfunction

Endothelial dysfunction is a key event in the development of vascular disease, and it precedes clinically obvious vascular pathology. Abnormal activation of the RhoA/Rho kinase (ROCK) pathway has been found to elevate vascular tone through unbalancing the production of vasodilating and vasoconstricting substances. Inhibition of the RhoA/ROCK pathway can prevent endothelial dysfunction in a variety of pathological conditions. This review, based on recent molecular, cellular, and animal studies, focuses on the current understanding of the ROCK pathway and its roles in endothelial dysfunction.

Long-term oral nitrate therapy is associated with adverse outcome in diabetic patients following elective percutaneous coronary intervention

BACKGROUND

To assess the impact of long-term oral nitrate therapy on clinical outcome following percutaneous coronary intervention (PCI) in patients with type II diabetes.

METHODS

The incidence of major adverse cardiovascular events (MACEs) following elective PCI for stable coronary artery disease was evaluated in 108 patients with type II diabetes (age 64.6±10.5 years, 67.7% men). Major adverse cardiovascular events were defined as the need for revascularization, non-fatal myocardial infarction or cardiovascular death. Multivariate Cox regression analysis was used to evaluate the predictive value of MACEs by clinical characteristics and the prescription of long-term nitrate therapy.

RESULTS

Isosorbide mononitrate (ISMN) was prescribed to 46 patients with an average dose of 44.3±15.2 mg/day. After a mean follow up of 25.3±25 months, 16 patients developed MACEs. Patients who received ISMN were more likely to suffer from MACEs (26.1% vs. 6.5%, P=0.01), mainly driven by a higher rate of acute coronary syndrome (13.0 vs 0%, P=0.01). Average daily dose of nitrate and other cardiovascular medication was not associated with MACEs. Multivariate Cox regression analysis revealed that prescription of only ISMN (Hazard Ratio 3.09, 95% CI 1.10-10.21, P=0.04) was an independent predictor for the development of MACEs.

CONCLUSION

Long-term oral nitrate therapy was associated with MACEs following elective coronary artery revascularization by PCI in patients with type II diabetes.

Peroxynitrite mediates diabetes-induced endothelial dysfunction: possible role of Rho kinase activation

Endothelial dysfunction is characterized by reduced bioavailability of NO due to its inactivation to form peroxynitrite or reduced expression of eNOS. Here, we examine the causal role of peroxynitrite in mediating diabetes-induced endothelial dysfunction. Diabetes was induced by STZ-injection, and rats received the peroxynitrite decomposition catalyst (FeTTPs, 15 mg/Kg/day) for 4 weeks. Vasorelaxation to acetylcholine, oxidative-stress markers, RhoA activity, and eNOS expression were determined. Diabetic coronary arteries showed significant reduction in ACh-mediated maximal relaxation compared to controls. Diabetic vessels showed also significant increases in lipid-peroxides, nitrotyrosine, and active RhoA and 50% reduction in eNOS mRNA expression. Treatment of diabetic animals with FeTTPS blocked these effects. Studies in aortic endothelial cells show that high glucose or peroxynitrite increases the active RhoA kinase levels and decreases eNOS expression and NO levels, which were reversed with blocking peroxynitrite or Rho kinase. Together, peroxynitrite can suppress eNOS expression via activation of RhoA and hence cause vascular dysfunction.

The promise of cell-based therapies for diabetic complications: challenges and solutions

The discovery of endothelial progenitor cells (EPCs) in human peripheral blood advanced the field of cell-based therapeutics for many pathological conditions. Despite the lack of agreement about the existence and characteristics of EPCs, autologous EPC populations represent a novel treatment option for complications requiring therapeutic revascularization and vascular repair. Patients with diabetic complications represent a population of patients that may benefit from cellular therapy yet their broadly dysfunctional cells may limit the feasibility of this approach. Diabetic EPCs have decreased migratory prowess and reduced proliferative capacity and an altered cytokine/growth factor secretory profile that can accelerate deleterious repair mechanisms rather than support proper vascular repair. Furthermore, the diabetic environment poses additional challenges for the autologous transplantation of cells. The present review is focused on correcting diabetic EPC dysfunction and the challenges involved in the application of cell-based therapies for treatment of diabetic vascular complications. In addition, ex vivo and in vivo functional manipulation(s) of EPCs to overcome these hurdles are discussed.

Endothelial progenitor dysfunction in the pathogenesis of diabetic retinopathy: treatment concept to correct diabetes-associated deficits

Progressive obliteration of the retinal microvessels is a characteristic of diabetic retinopathy and the resultant retinal ischemia can lead to sight-threatening macular edema, macular ischemia and ultimately preretinal neovascularization. Bone marrow derived endothelial progenitor cells (EPCs) play a critical role in vascular maintenance and repair. There is still great debate about the most appropriate markers that define an EPC. EPCs can be isolated using cell sorting by surface phenotype selection or in vitro cell culture. For freshly isolated cells, EPC cell sorting is heavily dependent on the surface markers used; EPCs can also be isolated by in vitro propagation of heterogeneous mixtures of cells in culture using adhesion to specific substrates and cell growth characteristics. in vitro isolation enables consistent reproducibility and using this approach at least two distinct types of EPCs with different angiogenic properties have been identified from adult peripheral and umbilical cord blood; early EPCs (eEPCs) and late outgrowth endothelial progenitor cells (OECs). Emerging studies demonstrate the potential of these cells in revascularization of ischemic/injured retina in animal models of retinal disease. Since ischemic retinopathies are leading causes of blindness, they are a potential disease target for EPC-based therapy. In this chapter, we summarize the current knowledge about EPCs and discuss the possibility of cellular therapy for treatment of diabetic macular ischemia and the vasodegenerative phase of diabetic retinopathy. We also report current pharmacological options that can be utilized to correct diabetes associated defects in EPCs so as to enhance the therapeutic utility of these cells.

Evidence against tetrahydrobiopterin depletion of vascular tissue exposed to nitric oxide/superoxide or nitroglycerin

Several cardiovascular disorders, including atherosclerosis and tolerance to the antianginal drug nitroglycerin (GTN), may be associated with the generation of superoxide anions, which react with nitric oxide (NO) to yield peroxynitrite. According to a widely held view, oxidation of tetrahydrobiopterin (BH(4)) by peroxynitrite causes uncoupling of endothelial NO synthase (eNOS), resulting in reduced NO bioavailability and endothelial dysfunction under conditions of oxidative stress. In this study we determined the levels of reduced biopterins and endothelial function in cultured cells exposed to peroxynitrite and GTN as well as in blood vessels isolated from GTN-tolerant guinea pigs and rats. BH(4) was rapidly oxidized by peroxynitrite and 3-morpholino sydnonimine (SIN-1) in buffer, but this was prevented by glutathione and not observed in endothelial cells exposed to SIN-1 or GTN. Prolonged treatment of the cells with 0.1 mM GTN caused slow N(G)-nitro-l-arginine-sensitive formation of reactive oxygen species without affecting eNOS activity. Endothelial function and BH(4)/BH(2) levels were identical in blood vessels of control and GTN-tolerant animals. Our results suggest that peroxynitrite-triggered BH(4) oxidation does not occur in endothelial cells or GTN-exposed blood vessels. GTN seems to trigger minor eNOS uncoupling that is unrelated to BH(4) depletion and without observable consequence on eNOS function.

Vascular dysfunction in retinopathy-an emerging role for arginase

Retinal neovascularization is a leading cause of visual disability. Retinal diseases involving neovascularization all follow the same progression, beginning with vascular inflammatory reactions and injury of the vascular endothelium and ending with neovascularization, fibrosis and retinal detachment. Understanding the mechanisms underlying this process is critical for its prevention and treatment. Research using retinopathy models has revealed that the NOX2 NADPH oxidase has a key role in inducing production of reactive oxygen species and angiogenic cytokines and causing vascular inflammatory reactions and neovascularization. This prospective review addresses the potential role of the urea/ornithine pathway enzyme arginase in this process. Studies of peripheral vessels isolated from diabetic animals have shown that increased arginase activity causes vascular endothelial cell dysfunction by decreasing availability of l-arginine to endothelial cell nitric oxide synthase which decreases nitric oxide bioavailability and increases oxidative stress. Increasing arginase activity also increases formation of polyamines and proline, which can induce cell growth and fibrosis. Studies in models of retinopathy show that increases in oxidative stress and signs of vascular inflammation are correlated with increases in arginase activity and arginase 1 expression and that decreasing arginase expression or inhibiting its activity blocks these effects. Furthermore, the induction of arginase during retinopathy is blocked by knocking out NOX2 or inhibiting NADPH oxidase activity. These observations suggest that NADPH oxidase-induced activation of the arginase pathway has a key role in causing retinal vascular dysfunction during retinopathy. Limiting the actions of arginase could provide a new strategy for treating this potentially blinding condition.

Role of the general base Glu-268 in nitroglycerin bioactivation and superoxide formation by aldehyde dehydrogenase-2

Mitochondrial aldehyde dehydrogenase-2 (ALDH2) plays an essential role in nitroglycerin (GTN) bioactivation, resulting in formation of NO or a related activator of soluble guanylate cyclase. ALDH2 denitrates GTN to 1,2-glyceryl dinitrate and nitrite but also catalyzes reduction of GTN to NO. To elucidate the relationship between ALDH2-catalyzed GTN bioconversion and established ALDH2 activities (dehydrogenase, esterase), we compared the function of the wild type (WT) enzyme with mutants lacking either the reactive Cys-302 (C302S) or the general base Glu-268 (E268Q). Although the C302S mutation led to >90% loss of all enzyme activities, the E268Q mutant exhibited virtually unaffected rates of GTN denitration despite low dehydrogenase and esterase activities. The nucleotide co-factor NAD caused a pronounced increase in the rates of 1,2-glyceryl dinitrate formation by WT-ALDH2 but inhibited the reaction catalyzed by the E268Q mutant. GTN bioactivation measured as activation of purified soluble guanylate cyclase or release of NO in the presence of WT- or E268Q-ALDH2 was markedly potentiated by superoxide dismutase, suggesting that bioavailability of GTN-derived NO is limited by co-generation of superoxide. Formation of superoxide was confirmed by determination of hydroethidine oxidation that was inhibited by superoxide dismutase and the ALDH2 inhibitor chloral hydrate. E268Q-ALDH2 exhibited approximately 50% lower rates of superoxide formation than the WT enzyme. Our results suggest that Glu-268 is involved in the structural organization of the NAD-binding pocket but is not required for GTN denitration. ALDH2-catalyzed superoxide formation may essentially contribute to oxidative stress in GTN-exposed blood vessels.

Endothelial dysfunction and diabetes: roles of hyperglycemia, impaired insulin signaling and obesity

Endothelial dysfunction comprises a number of functional alterations in the vascular endothelium that are associated with diabetes and cardiovascular disease, including changes in vasoregulation, enhanced generation of reactive oxygen intermediates, inflammatory activation, and altered barrier function. Hyperglycemia is a characteristic feature of type 1 and type 2 diabetes and plays a pivotal role in diabetes-associated microvascular complications. Although hyperglycemia also contributes to the occurrence and progression of macrovascular disease (the major cause of death in type 2 diabetes), other factors such as dyslipidemia, hyperinsulinemia, and adipose-tissue-derived factors play a more dominant role. A mutual interaction between these factors and endothelial dysfunction occurs during the progression of the disease. We pay special attention to the possible involvement of endoplasmic reticulum stress (ER stress) and the role of obesity and adipose-derived adipokines as contributors to endothelial dysfunction in type 2 diabetes. The close interaction of adipocytes of perivascular adipose tissue with arteries and arterioles facilitates the exposure of their endothelial cells to adipokines, particularly if inflammation activates the adipose tissue and thus affects vasoregulation and capillary recruitment in skeletal muscle. Hence, an initial dysfunction of endothelial cells underlies metabolic and vascular alterations that contribute to the development of type 2 diabetes.

The enigma of nitroglycerin bioactivation and nitrate tolerance: news, views and troubles

Nitroglycerin (glyceryl trinitrate; GTN) is the most prominent representative of the organic nitrates or nitrovasodilators, a class of compounds that have been used clinically since the late nineteenth century for the treatment of coronary artery disease (angina pectoris), congestive heart failure and myocardial infarction. Medline lists more than 15 000 publications on GTN and other organic nitrates, but the mode of action of these drugs is still largely a mystery. In the first part of this article, we give an overview on the molecular mechanisms of GTN biotransformation resulting in vascular cyclic GMP accumulation and vasodilation with focus on the role of mitochondrial aldehyde dehydrogenase (ALDH2) and the link between the ALDH2 reaction and activation of vascular soluble guanylate cyclase (sGC). In particular, we address the identity of the bioactive species that activates sGC and the potential involvement of nitrite as an intermediate, describe our recent findings suggesting that ALDH2 catalyses direct 3-electron reduction of GTN to NO and discuss possible reaction mechanisms. In the second part, we discuss contingent processes leading to markedly reduced sensitivity of blood vessels to GTN, referred to as vascular nitrate tolerance. Again, we focus on ALDH2 and describe the current controversy on the role of ALDH2 inactivation in tolerance development. Finally, we emphasize some of the most intriguing, in our opinion, unresolved puzzles of GTN pharmacology that urgently need to be addressed in future studies.

Diabetes-induced coronary vascular dysfunction involves increased arginase activity

Increases in arginase activity have been reported in a variety of disease conditions characterized by vascular dysfunction. Arginase competes with NO synthase for their common substrate arginine, suggesting a cause and effect relationship. We tested this concept by experiments with streptozotocin diabetic rats and high glucose (HG)-treated bovine coronary endothelial cells (BCECs). Our studies showed that diabetes-induced impairment of vasorelaxation to acetylcholine was correlated with increases in reactive oxygen species and arginase activity and arginase I expression in aorta and liver. Treatment of diabetic rats with simvastatin (5 mg/kg per day, subcutaneously) or L-citrulline (50 mg/kg per day, orally) blunted these effects. Acute treatment of diabetic coronary arteries with arginase inhibitors also reversed the impaired vasodilation to acetylcholine. Treatment of BCECs with HG (25 mmol/L, 24 hours) also increased arginase activity. This effect was blocked by treatment with simvastatin (0.1 micromol/L), the Rho kinase inhibitor Y-27632 (10 micromol/L), or L-citrulline (1 mmol/L). Superoxide and active RhoA levels also were elevated in HG-treated BCECs. Furthermore, HG significantly diminished NO production in BCECs. Transfection of BCECs with arginase I small interfering RNA prevented the rise in arginase activity in HG-treated cells and normalized NO production, suggesting a role for arginase I in reduced NO production with HG. These results indicate that increased arginase activity in diabetes contributes to vascular endothelial dysfunction by decreasing L-arginine availability to NO synthase.

Increased superoxide production in nitrate tolerance is associated with NAD(P)H oxidase and aldehyde dehydrogenase 2 downregulation

Chronic administration of nitroglycerin (NTG) induces nitrate tolerance. Among possible underlying mechanisms, increased vascular production of reactive oxygen species (ROS) has emerged as a principal mechanism. Using cell culture and animal models of nitrate tolerance, we aimed to assess the impact of nitrates on NAD(P)H oxidases and aldehyde dehydrogenase 2 (ALDH2) expression. Rats and vascular smooth muscle cells were treated with NTG. Vascular reactivity was assessed by isometric tension studies. Superoxide was detected by dihydroethidium staining. Gene expression was measured by real-time polymerase chain reaction. NAD(P)H oxidase activity was measured using lucigenin-enhanced chemiluminescence. ALDH activity was measured biochemically, and NO consumption electrochemically. Nitrate tolerance was induced in rats by treatment with NTG for 3 days, and detected as impaired endothelium-dependent and -independent relaxation of aortic segments. Although superoxide production was increased in all aortic layers, expression of nox1, nox2 and nox4 was significantly decreased. Similarly, in vascular smooth muscle cells exposed to NTG for 6-24 h, NAD(P)H oxidase activity was increased, in spite of nox1 downregulation. In addition, expression and activity of ALDH-2 was decreased in nitrate-tolerant rings. Furthermore, exogenous addition of ALDH decreased superoxide generation in vitro and attenuated NO consumption in vascular smooth muscle cell homogenates. Our data suggest that in nitrate tolerance, activation of nox enzymes more than compensates for their downregulation, resulting in a net increase in superoxide and NO consumption. Furthermore, reduced ALDH-2 activity and expression leads to decreased NTG bioconversion. Therefore, both mechanisms reduce NO availability and impair vasorelaxation.

Therapeutic use of citrulline in cardiovascular disease

L-citrulline is the natural precursor of L-arginine, substrate for nitric oxide synthase (NOS) in the production of NO. Supplemental administration L-arginine has been shown to be effective in improving NO production and cardiovascular function in cardiovascular diseases associated with endothelial dysfunction, such as hypertension, heart failure, atherosclerosis, diabetic vascular disease and ischemia-reperfusion injury, but the beneficial actions do not endure with chronic therapy. Substantial intestinal and hepatic metabolism of L-arginine to ornithine and urea by arginase makes oral delivery very ineffective. Additionally, all of these disease states as well as supplemental L-arginine enhance arginase expression and activity, thus reducing the effectiveness of L-arginine therapy. In contrast, L-citrulline is not metabolized in the intestine or liver and does not induce tissue arginase, but rather inhibits its activity. L-citrulline entering the kidney, vascular endothelium and other tissues can be readily converted to L-arginine, thus raising plasma and tissue levels of L-arginine and enhancing NO production. Supplemental L-citrulline has promise as a therapeutic adjunct in disease states associated with L-arginine deficiencies.

Once daily therapy with isosorbide-5-mononitrate causes endothelial dysfunction in humans: evidence of a free-radical-mediated mechanism

OBJECTIVES

The aim of the study was to determine if isosorbide-5-mononitrate (IS-5-MN) 120 mg, taken once daily for 7 days, is associated with evidence of endothelial dysfunction and whether this effect is determined by increased free radical production.

BACKGROUND

Tolerance to nitroglycerin is associated with increased free radical production and abnormal endothelial function. To date, no data is available concerning the effect of IS-5-MN, administered in clinically employed dosages, on endothelial function in humans.

METHODS

A total of 19 healthy volunteers were randomized in a double-blind fashion to therapy with IS-5-MN (120 mg once daily) or placebo. After 7 days of treatment, forearm blood flow responses to acetylcholine (Ach; 7.5, 15, and 30 microg/min) and N-monomethyl-L-arginine (L-NMMA; 1, 2, and 4 mumol/min) were measured. In a separate study, after 7 days of therapy with IS-5-MN 120 mg once daily, the responses to Ach were assessed during intra-arterial coinfusion of vitamin C (24 mg/min) or saline.

RESULTS

As compared with placebo, IS-5-MN caused significant blunting of the responses to both Ach (peak responses: placebo 127 +/- 31%; IS-5-MN 52 +/- 24%) and L-NMMA (peak responses: placebo 41 +/- 5%; IS-5-MN 22 +/- 8%). Vitamin C completely restored the forearm blood flow responses to Ach (peak responses: vitamin C 180 +/- 33%; saline 107 +/- 17%).

CONCLUSIONS

We document for the first time that IS-5-MN impairs endothelial function in humans in vivo. Suggesting a role of oxygen free radicals, nitrate-induced abnormalities in endothelium-dependent vasomotor responses were reversed by the antioxidant vitamin C.

Ramipril treatment protects against nitrate-induced oxidative stress in eNOS-/- mice: An implication of the NADPH oxidase pathway

The development of nitrate tolerance has been found to be associated with vascular production of superoxide anion (O2-*), generated mainly by the eNOS and NADPH oxidase pathways. The aim of our study was to investigate whether long-term angiotensin-converting enzyme inhibition by ramipril is able to protect against nitrate tolerance in the aortas of eNOS-deficient (eNOS-/-) mice and to assess the implication of the NADPH oxidase pathway. Therefore, 3 types of treatment were given to wild-type (WT) and eNOS-/- mice: group 1 received ramipril for 5 weeks and a co-treatment with ramirpil plus nitroglycerine (NTG) during the last 4 days, group 2 received only NTG, and group 3 served as control. Relaxations to NTG (0.1 nmol/L to 0.1 mmol/L) were determined on U44619, a thromboxane analogue, precontracted rings, and O2-* production were assessed on aorta homogenates with the lucigenin-enhanced chemiluminescence technique. Cyclic guanosine monophosphate and reverse-transcriptase-polymerase chain reaction analyses were performed on whole mouse aortas. In WT group 2, the concentration-effect curves to NTG were significantly shifted to the right: the pD2 was 6.16 +/- 0.17 (n = 6) vs 6.81 +/- 0.10 (n = 6) in WT group 3 (not exposed to NTG; P < 0.05) and O2-* production was enhanced from 100% +/- 11% (n = 9) to 191% +/- 21% (n = 6; P < 0.01). In contrast, in WT group 1, the rightward shift was abolished: the pD2 value was 6.73 +/- 0.13 (n = 6; NS vs group 3 WT) and O2-* production was 117% +/- 6% (n = 7; NS vs group 3 WT). In eNOS groups 1 and 3, similar data were observed: the pD2 values were 7.58 +/- 0.08 and 7.38 +/- 0.11 (NS) vs 6.89 +/- 0.20 in eNOS group 2 (n = 6; P < 0.01). In the WT mice aortas, ramipril treatment significantly increased the cyclic guanosine monophosphate levels (reflecting nitric oxide availability), which returned to control values after in vivo co-treatment with a bradykinin BK2 antagonist (Icatibant). In both strains, candesartan, an AT1 blocker, was also able to protect against the development of nitrate tolerance. Moreover, before NTG exposure, ramipril treatment decreased p22phox and gp91phox (essential NADPH oxidase subunits) mRNA expression in aortas from both mice strains. In conclusion, long-term ramipril treatment in mice protects against the development of nitrate tolerance by counteracting NTG-induced increase in O2 production, which involves a direct interaction with the NADPH oxidase pathway and seems to be completely independent of the eNOS pathway.

Effect of resveratrol on nitrate tolerance in isolated human internal mammary artery

The present study aims to examine whether resveratrol, a natural antioxidant present in red wine, restores the tolerance to nitroglycerin (GTN) on isolated human internal mammary artery (IMA), using an in vitro model of nitrate tolerance. IMA rings were obtained from 53 male patients undergoing coronary bypass operation. Nitrate tolerance was induced by incubating the artery ring with 100 microM GTN for 90 minutes. Concentration-response curves to GTN (10(-9) to 10(-4) M) were obtained on IMA rings precontracted with noradrenaline. A low concentration (5 microM) of lucigenin was used as a tool to measure superoxide production in IMA segments. GTN produced concentration-dependent relaxation in isolated human IMA rings. Preexposure of artery rings to GTN reduced the relaxations to GTN [E(max) values: 105 +/- 2% and 76 +/- 3%, n = 10 to 12, P < 0.05; EC(50) values (-log M): 6.72 +/- 0.05 and 4.95 +/- 0.06, P < 0.05, respectively]. Relaxation to sodium nitroprusside remained unchanged. Diminished relaxation to GTN is partially restored after removing endothelium or L(G)-nitro-L-arginine (L-NOARG, 10 M) or superoxide dismutase (20 and 200 U/mL) or catalase (200 U/mL) pretreatments. Pretreatments with resveratrol (1, 10, and 20 microM) for 20 minutes relatively improved the reduced relaxation to GTN in tolerant IMA rings. Coadministration of L-NOARG with resveratrol did not abolish the beneficial effect of resveratrol on nitrate tolerance. The inhibitory effect of resveratrol on GTN-induced tolerance was not abolished in arterial rings without endothelium. Exposure to GTN increased superoxide production in IMA segments with endothelium. Endothelium denudation, L-NOARG, or superoxide dismutase pretreatments markedly inhibited the increased superoxide production in tolerant arteries. Resveratrol (1 and 10 microM) almost completely abolished basal or NAD(P)H-stimulated superoxide production in tolerant and nontolerant arteries. Vascular tolerance to GTN, in in vitro tolerant human IMA rings, can be induced by endothelial superoxide anions. Resveratrol partially restored the reduced relaxation to GTN by inhibiting NAD(P)-derived superoxide production in endothelium.

Nitroglycerin attenuates human endothelial progenitor cell differentiation, function, and survival

Endothelial progenitor cells (EPCs) participate in angiogenesis and the response to chronic ischemia. Risk factors and cardiovascular disease attenuate EPC number, function, and survival. Continuous therapy with nitroglycerin (glyceryl trinitrate; GTN) is associated with increased vascular oxidative stress, leading to nitrate tolerance and endothelial dysfunction. Thus, GTN therapy may also affect EPCs. The purpose of this study was to determine whether continuous exposure to GTN in vivo or during ex vivo expansion affects the circulating number and functional characteristics of human EPCs. To determine the effects of continuous in vivo GTN exposure, EPCs isolated from 28 healthy males before and after receiving 0.6 mg/h GTN (n=17) or no treatment (n=11) for 1 week were expanded for 6 days and compared. To determine the effects of continuous ex vivo GTN exposure, EPCs isolated before randomization were expanded for 6 days in medium supplemented with 100 nM, 300 nM, or 1 microM GTN. EPCs expanded without GTN served as controls (n=10). In vivo, GTN exposure significantly increased the percentage of circulating cells expressing the EPC marker CD34 and increased the susceptibility of expanded EPCs to apoptosis but had no impact on the phenotypic differentiation or migration of EPCs. Ex vivo, GTN exposure increased apoptosis while decreasing phenotypic differentiation, migration, and mitochondrial dehydrogenase activity of EPCs, compared with EPCs expanded in the absence of GTN. Taken together, these results suggest that continuous GTN therapy might impair EPC-mediated processes, an effect that could be detrimental in the setting of ischemic cardiovascular disease.

Explaining the phenomenon of nitrate tolerance

During the last century, nitroglycerin has been the most commonly used antiischemic and antianginal agent. Unfortunately, after continuous application, its therapeutic efficacy rapidly vanishes. Neurohormonal activation of vasoconstrictor signals and intravascular volume expansion constitute early counter-regulatory responses (pseudotolerance), whereas long-term treatment induces intrinsic vascular changes, eg, a loss of nitrovasodilator-responsiveness (vascular tolerance). This is caused by increased vascular superoxide production and a supersensitivity to vasoconstrictors secondary to a tonic activation of protein kinase C. NADPH oxidase(s) and uncoupled endothelial nitric oxide synthase have been proposed as superoxide sources. Superoxide and vascular NO rapidly form peroxynitrite, which aggravates tolerance by promoting NO synthase uncoupling and inhibition of soluble guanylyl cyclase and prostacyclin synthase. This oxidative stress concept may explain why radical scavengers and substances, which reduce oxidative stress indirectly, are able to relieve tolerance and endothelial dysfunction. Recent work has defined a new tolerance mechanism, ie, an inhibition of mitochondrial aldehyde dehydrogenase, the enzyme that accomplishes bioactivation of nitroglycerin, and has identified mitochondria as an additional source of reactive oxygen species. Nitroglycerin-induced reactive oxygen species inhibit the bioactivation of nitroglycerin by thiol oxidation of aldehyde dehydrogenase. Both mechanisms, increased oxidative stress and impaired bioactivation of nitroglycerin, can be joined to provide a new concept for nitroglycerin tolerance and cross-tolerance. The consequences of these processes for the nitroglycerin downstream targets soluble guanylyl cyclase, cGMP-dependent protein kinase, cGMP-degrading phosphodiesterases, and toxic side effects contributing to endothelial dysfunction, such as inhibition of prostacyclin synthase, are discussed in this review.

Roles of superoxide, peroxynitrite, and protein kinase C in the development of tolerance to nitroglycerin

A current hypothesis states that tolerance to nitroglycerin (GTN) involves increased formation of superoxide (O2*-). Studies showing that inhibitors of protein kinase C (PKC) prevent tolerance to GTN suggest the involvement of PKC activation, which can also increase O2*-. We examined the roles of O2*-, peroxynitrite (ONOO-), and PKC activation in GTN tolerance. Pre-exposure of rat aortic rings to GTN (5 x 10(-4) M) for 2 h caused tolerance to the vasodilating effect of GTN, as evidenced by a substantial rightward shift of GTN concentration-relaxation curves. This shift was reduced by treatment of the rings with the antioxidants uric acid, vitamin C, or tempol or the PKC inhibitor chelerythrine. We also found that O2*- generation via xanthine/xanthine oxidase in the bath induced tolerance to GTN. However, responses to nitroprusside were not affected. In vivo tolerance produced in rats by 3-day i.v. infusion of GTN was also almost completely prevented by coinfusion of tempol. In bovine aortic endothelial cells (EC), addition of GTN produced a marked increase in tyrosine nitrosylation, indicating increased ONOO- formation. This action was blocked by prior treatment with uric acid, superoxide dismutase, NG-nitro-L-arginine methyl ester, or chelerythrine. We also demonstrated that GTN translocates the alpha- and epsilonPKC isoforms in EC. However, PKCzeta was not affected by GTN treatment. In conclusion, tolerance to GTN involves enhanced production of O2*- and ONOO- and activation of NO synthase. Furthermore, sustained activation of alpha- and epsilonPKC isozymes in EC by GTN may play a role in development of tolerance.

Melatonin inhibits nitrate tolerance in isolated coronary arteries

(1) The present study was designed to test the hypothesis that melatonin inhibits nitrate tolerance in coronary arteries. (2) Rings of porcine coronary arteries were suspended in organ chambers for isometric tension recording. Nitrate tolerance was induced by incubating the tissues with nitroglycerin (10(-4) M) for 90 min, followed by repeated rinsing for 1 h. Control rings that had not been exposed previously to nitroglycerin, but were otherwise treated identically, were studied simultaneously. The rings were contracted with U46619 (1-3 x 10(-9) M) and concentration-response curves to nitroglycerin (10(-9)-10(-4) M) were obtained. (3) Nitrate tolerance was evident by a 15- to 20-fold rightward shift in the concentration-response curve to nitroglycerin in rings with and without endothelium exposed previously to the drug for 90 min. Addition of melatonin (10(-9)-10(-7) M) to the organ chamber during the 90-min incubation period with nitroglycerin partially inhibited nitrate tolerance in coronary arteries with intact endothelium; however, melatonin had no effect on nitrate tolerance in coronary arteries without endothelium. (4) The effect of melatonin on nitrate tolerance in coronary arteries with endothelium was abolished by the melatonin receptor antagonist, S20928 (10(-6) M). In contrast to melatonin, the selective MT(3)-melatonin receptor agonist, 5-MCA-NAT (10(-8)-10(-7) M), had no effect on nitrate tolerance in coronary arteries. (5) The results demonstrate that melatonin, acting via specific melatonin receptors, inhibits nitrate tolerance in coronary arteries and that this effect is dependent on the presence of the vascular endothelium.

Experimental diabetes causes breakdown of the blood-retina barrier by a mechanism involving tyrosine nitration and increases in expression of vascular endothelial growth factor and urokinase plasminogen activator receptor

The purpose of these experiments was to determine the specific role of reactive oxygen species (ROS) in the blood-retinal barrier (BRB) breakdown that characterizes the early stages of vascular dysfunction in diabetes. Based on our data showing that high glucose increases nitric oxide, superoxide, and nitrotyrosine formation in retinal endothelial cells, we hypothesized that excess formation of ROS causes BRB breakdown in diabetes. Because ROS are known to induce increases in expression of the well-known endothelial mitogen and permeability factor vascular endothelial growth factor (VEGF) we also examined their influence on the expression of VEGF and its downstream target urokinase plasminogen activator receptor (uPAR). After 2 weeks of streptozotocin-induced diabetes, analysis of albumin leakage confirmed a prominent breakdown of the BRB. This permeability defect was correlated with significant increases in the formation of nitric oxide, lipid peroxides, and the peroxynitrite biomarker nitrotyrosine as well as with increases in the expression of VEGF and uPAR. Treatment with a nitric oxide synthase inhibitor (N-omega-nitro-L-arginine methyl ester, 50 mg/kg/day) or peroxynitrite scavenger (uric acid, 160 mg/kg/day) blocked the breakdown in the BRB and prevented the increases in formation of lipid peroxides and tyrosine nitration as well as the increases in expression of VEGF and uPAR. Taken together, these data indicate that early diabetes causes breakdown of the BRB by a mechanism involving the action of reactive nitrogen species in promoting expression of VEGF and uPAR.

Experimental diabetes causes breakdown of the blood-retina barrier by a mechanism involving tyrosine nitration and increases in expression of vascular endothelial growth factor and urokinase plasminogen activator receptor

The purpose of these experiments was to determine the specific role of reactive oxygen species (ROS) in the blood-retinal barrier (BRB) breakdown that characterizes the early stages of vascular dysfunction in diabetes. Based on our data showing that high glucose increases nitric oxide, superoxide, and nitrotyrosine formation in retinal endothelial cells, we hypothesized that excess formation of ROS causes BRB breakdown in diabetes. Because ROS are known to induce increases in expression of the well-known endothelial mitogen and permeability factor vascular endothelial growth factor (VEGF) we also examined their influence on the expression of VEGF and its downstream target urokinase plasminogen activator receptor (uPAR). After 2 weeks of streptozotocin-induced diabetes, analysis of albumin leakage confirmed a prominent breakdown of the BRB. This permeability defect was correlated with significant increases in the formation of nitric oxide, lipid peroxides, and the peroxynitrite biomarker nitrotyrosine as well as with increases in the expression of VEGF and uPAR. Treatment with a nitric oxide synthase inhibitor (N-omega-nitro-L-arginine methyl ester, 50 mg/kg/day) or peroxynitrite scavenger (uric acid, 160 mg/kg/day) blocked the breakdown in the BRB and prevented the increases in formation of lipid peroxides and tyrosine nitration as well as the increases in expression of VEGF and uPAR. Taken together, these data indicate that early diabetes causes breakdown of the BRB by a mechanism involving the action of reactive nitrogen species in promoting expression of VEGF and uPAR.

Nitric oxide synthase expressions in mice skeletal muscle subjected to ischemia/reperfusion injury

Nitric oxide synthase (NOS) expressions in skeletal muscle subjected to ischemia/reperfusion (I/R) were studied using a hind limb tourniquet ischemia model in mice. A rubber band was applied to a hind limb for 3 h under isoflurane anesthesia followed by 1 or 4 h of reperfusion. Increased NADPH diaphorase activity and NOS immunoreactivity were histochemically detected in the cells of muscle that had been subjected to I/R. The results of RT-PCR of the muscle subjected to I/R showed that NOS mRNA expressions were not significantly increased until 4 h after the start of reperfusion. Since there was no significant difference between histochemical findings or between water contents of the hind limbs or organs in interleukin (IL)-6-deficient mice and the wild-type mice, IL-6 may not be involved in the early stage of I/R muscle injury such as that in this model. O(2)(-) production in the cells of muscle that had been subjected to I/R was observed using an in situ detection method with hydroethidine, and the O(2)(-) was inhibited by intravenous administration of L-NAME or L-NMMA, but not L-NIL, 30 min before tourniquet release. Further study is needed to evaluate the role of O(2)(-) produced by constitutive NOS in muscle subjected to I/R in the pathophysiology of tourniquet shock.

Effect of folic acid on nitrate tolerance in healthy volunteers: differences between arterial and venous circulation

This study investigated whether oral supplemental folic acid can prevent the development of nitrate tolerance and whether it has different effects on the arterial and venous systems. Twenty-four healthy male volunteers received either placebo or folic acid (10 mg/d) for 14 days. Additionally, all subjects underwent concurrent transdermal nitroglycerin therapy for 7 days. Venous occlusion forearm strain gauge plethysmography measured arterial and venous responses to sublingual nitroglycerin before and after treatment. Both arterial and venous responses were blunted in the placebo group after transdermal nitroglycerin. Folic acid prevented the development of nitrate tolerance in arteries but had no effect in veins.

Mechanisms underlying nitrate-induced endothelial dysfunction: insight from experimental and clinical studies

The hemodynamic and anti-ischemic effects of nitroglycerin (NTG) are rapidly blunted due to the development of nitrate tolerance. With initiation of nitroglycerin therapy one can detect neurohormonal activation and signs for intravascular volume expansion. These so called pseudotolerance mechanisms may compromise nitroglycerin's vasodilatory effects. Long-term treatment with nitroglycerin is also associated with a decreased responsiveness of the vasculature to nitroglycerin's vasorelaxant potency suggesting changes in intrinsic mechanisms of the tolerant vasculature itself may also contribute to tolerance. More recent experimental work defined new mechanisms of tolerance such as increased vascular superoxide production and increased sensitivity to vasoconstrictors secondary to an activation of the intracellular second messenger protein kinase C. As potential superoxide producing enzymes, the NADPH oxidase and the nitric oxide synthase have been identified. Nitroglycerin-induced stimulation of oxygen-derived free radicals together with NO derived from nitroglycerin may lead to the formation of peroxynitrite, which may be responsible for the development of tolerance as well as for the development of cross tolerance to endothelium-dependent vasodilators. The oxidative stress concept of tolerance and cross tolerance may explain why radical scavengers such as vitamin C or substances which reduce oxidative stress, such as ACE-inhibitors, AT1 receptor blockers or folic acid, are able to beneficially influence both tolerance and nitroglycerin-induced endothelial dysfunction. New aspects concerning the role of oxidative stress in nitrate tolerance and nitrate induced endothelial dysfunction and the consequences for the NO/cyclicGMP downstream target, the cGMP-dependent protein kinase will be discussed.

Effect of gender on endothelium-dependent dilation to bradykinin in human adipose microvessels

We examined the influence of gender and climacteric status, two coronary risk factors, on bradykinin (BK)-induced dilation in adipose arterioles from men and women of different ages [premenopausal women (Pre-W), postmenopausal women (Post-W), and similar aged men (Y-M and O-M), respectively]. We examined the responses from both omental (more closely associated with coronary disease) and subcutaneous fat. Tissues were obtained at surgery and cannulated (60 mmHg) for measurement of internal diameter. In vessels from omental tissue, dilation to BK was more sensitive in Pre-W than other groups, whereas in vessels from subcutaneous tissue, sensitivity to BK was greater in both Pre-W and Post-W compared with Y-M and O-M. Maximal dilation was similar among groups. Indomethacin (Indo; 10(-5) M) alone had no effect on dilation to BK in any groups, but Indo and N(omega)-nitro-L-arginine methyl ester (L-NAME; 10(-4) M) reduced dilation to BK in Pre-W more than in Y-M. L-NAME increased dilation to BK in subcutaneous fat from Y-M but had no effect in Post-W and O-M. Indo- and L-NAME-resistant dilation in all vessels was markedly reduced by 30 mM KCl. There was no difference in sodium nitroprusside-induced dilation among groups. We conclude that gender and climacteric state contribute to mechanisms of microvascular regulation in humans. Functional vascular differences in visceral and subcutaneous fat may underlie the proposed differential influence of these tissues on cardiovascular risk.

The effect of supplemental L-arginine on tolerance development during continuous transdermal nitroglycerin therapy

OBJECTIVES

This study was designed to assess the effect of oral L-arginine on the development of tolerance during continuous transdermal nitroglycerin (TD-GTN) therapy.

BACKGROUND

Continuous TD-GTN therapy leads to complete tolerance within 24 to 48 h. The mechanism(s) responsible for nitrate tolerance are unclear, but there is increasing evidence that nitroglycerin (GTN) leads to superoxide anion production. The trigger for this is unknown, but there is evidence that GTN alters nitric oxide synthase (NOS) function and also leads to reduced L-arginine availability at its site of action with NOS.

METHODS

Fourteen patients with stable angina pectoris and reproducible treadmill walking time (TWT) until the onset of moderate angina were studied in a placebo-controlled, crossover study. Transdermal GTN (0.4 mg/h) was applied daily for two periods of 5 to 10 days with the patch left in place for 24 h each day. Capsules containing L-arginine (700 mg) or placebo were administered four times daily during a period of TD-GTN therapy. Treadmill walking time was determined before and 4 h after study capsules on day 1 before TD-GTN to assess the effect of L-arginine on exercise performance. On the last day, TWT was determined at 0 h (24 h after TD-GTN and 9 h after study capsule) and 4 h after TD-GTN reapplication and study capsule. After a 5 to 10 day washout period, the study was repeated with the opposite study capsule.

RESULTS

Treadmill walking time until the onset of moderate angina was not influenced by the short-term administration of L-arginine. During continuous TD-GTN, the administration of L-arginine increased TWT 4 h and 24 h after patch application. This was significantly greater than TWT during administration of placebo capsules (p < 0.05).

CONCLUSIONS

The administration of L-arginine modified or prevented the development of nitrate tolerance during continuous TD-GTN therapy.

Oxidative stress and nitric oxide synthase in rat diabetic nephropathy: effects of ACEI and ARB

BACKGROUND

Angiotensin II (Ang II) can up-regulate nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase, whose product superoxide anion (O2-) can interact with nitric oxide (NO) to form peroxynitrite (ONOO-). We tested the hypothesis that Ang II subtype 1 (AT1) receptor activation enhances oxidative stress and nitrotyrosine deposition in the kidneys of rats with diabetes mellitus (DM).

METHODS

After two weeks of streptozotocin-induced DM, rats received either no treatment, an angiotensin-converting enzyme inhibitor (ACEI) or an angiotensin receptor blocker (ARB) for two weeks. At four weeks, renal expression of the p47phox component of NAD(P)H oxidase, endothelial nitric oxide synthase (eNOS), neuronal nitric oxide synthase (nNOS), and nitrotyrosine were evaluated by Western blot and immunohistochemistry and related to plasma lipid peroxidation products (LPO), hydrogen peroxide production in the kidney and 24-hour protein excretion.

RESULTS

Immunoreactive expression of p47phox and eNOS were increased in DM with an increase in plasma LPO, renal hydrogen peroxide production and nitrotyrosine deposition. Expression of nNOS was unaltered. Treatment with either ACEI or ARB prevented all these findings and also prevented significant microalbuminuria. The treatments did not affect the elevated blood sugar, nor did DM or its treatment affect the blood pressure or the creatinine clearance.

CONCLUSION

Early proteinuric diabetic nephropathy increases renal expression of the p47phox component of NAD(P)H oxidase and eNOS with increased indices of systemic and renal oxidative/nitrosative stress. An ACEI or an ARB prevents these changes and prevents the development of proteinuria, independent of blood pressure or blood sugar. This finding indicates a pathogenic role for AT1 receptors in the development of oxidative damage in the kidneys during early DM.

Folic acid prevents nitroglycerin-induced nitric oxide synthase dysfunction and nitrate tolerance: a human in vivo study

BACKGROUND

In healthy humans, continuous treatment with nitroglycerin (GTN) causes nitric oxide synthase dysfunction, probably through the reduced bioavailability of tetrahydrobiopterin. Recent studies proposed that folic acid is involved in the regeneration of tetrahydrobiopterin in different disease states. Therefore, we investigated whether folic acid administration would prevent this phenomenon. We also sought to determine if folic acid supplementation could prevent the development of tolerance to GTN.

METHODS AND RESULTS

On the first visit, 18 healthy male volunteers (aged 19 to 32 years) were randomized to receive either oral folic acid (10 mg once a day) or placebo for 1 week in a double-blind designed study. All subjects also received continuous transdermal GTN (0.6 mg/h). On the second visit, forearm blood flow was measured with venous occlusion strain gauge plethysmography in response to incremental infusions of acetylcholine (7.5, 15, and 30 microgram/min), N-monomethyl-L-arginine (1, 2, and 4 micromol/min), and GTN (11 and 22 nmol/min). Folic acid prevented GTN-induced endothelial dysfunction, as assessed by responses to intraarterial acetylcholine and N-monomethyl-L-arginine (P<0.01). Moreover, in the subjects treated with folic acid plus transdermal GTN, responses to intraarterial GTN were significantly greater than those observed after transdermal GTN plus placebo (P<0.05).

CONCLUSION

Our data demonstrate that supplemental folic acid prevents both nitric oxide synthase dysfunction induced by continuous GTN and nitrate tolerance in the arterial circulation of healthy volunteers. We hypothesize that the reduced bioavailability of tetrahydrobiopterin is involved in the pathogenesis of both phenomena. Our results confirm the view that oxidative stress contributes to nitrate tolerance.